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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: do { quit; } else { }
Date: Sun, 13 Apr 2025 18:22:42 -0700
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Thiago Adams <thiago.adams@gmail.com> writes:
> Em 4/13/2025 6:58 PM, Keith Thompson escreveu:
>> Kaz Kylheku <643-408-1753@kylheku.com> writes:
>>> On 2025-04-13, Michael S <already5chosen@yahoo.com> wrote:
>>>> The funny thing is that in original C prefix form of [] indexing was
>>>> equivalent to the postfix form.  May be, it still is in C23, I didn't
>>>> try to look in the docs or test.
>>>
>>> Are you thinking of a[i] being *(a + i) being i[a] due to commutativity?
>>>
>>> That's really not a useful feature. Actually right down to a + i being
>>> commutative when a is a pointer. It would be fine if only the
>>> <pointer> + <integer> combination were allowed.
>> Agreed.  The feature goes back to B, which was largely untyped, and
>> there wasn't a strong distinction between integers and pointers.  An
>> expression like 5[6] would refer to a memory location, and depending
>> on the system it might even work.
>> The transition from B to C was an opportunity to restrict the
>> ordering
>> of the operands, but it wasn't taken.
>
> Proposal for c2y
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3517.htm
>
> " A postfix expression followed by an expression in square brackets [
> ] is a subscripted designation of an element of an array. The use of
> this operator with the first operand of integer type **is an
> obsolescent feature.**"
>
> and make M[-1]  a constrain

In the proposal, a negative index is a constraint violation only if it's
an integer constant expression, and only if the prefix is the name of an
array object.  ptr[-1] is still valid, for example:

    int arr[10];
    int *ptr = arr+1;
    ptr[-1];

You probably meant to say this; your post appears to have been cut off.

But yes, I'm glad to see swapping the operands being made obsolescent.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */